1. Field of the Invention
This invention relates to Josephson junction devices, and more particularly to such devices having superconducting electrodes containing intermetallic regions.
2. Description of the Prior Art
Josephson junction devices are well known in the art as can be seen by referring to J. Matisoo, Proceedings of the IEEE, Vol. 55, No. 2, February, 1967, pp. 172-180; U.S. Pat. No. 3,281,609; and J. Matisoo, Journal of Applied Physics, Vol. 40, No. 4, pp. 1813-1820, March 15, 1969. In general, such devices include superconducting electrodes and a tunnel barrier therebetween. The tunnel barrier is usually an oxide of the base electrode, although other materials and oven vacuum will serve as well. The device is characterized by a very thin tunnel barrier, of the order of 3-50 angstroms, through which Josephson tunneling current can flow at zero voltage. These devices have two tunneling states; a pair tunneling state in which Josephson current exists at zero junction voltage and a single particle tunneling state in which current exists at a finite voltage. The presence of two stable states enables these devices to be capable of logical memory functions.
Fabrication of usable Josephson junction devices is generally a difficult problem. The tunneling resistance must be low in order to have high maximum Josephson currents and the devices must show good stability over repeated thermal cycling. In addition, the devices must have no transformations (material property changes) when being used or stored.
Because materials such as lead have high super-conducting-to-normal transition temperature, they are desirable for use as superconducting electrodes. However, hillock formation occurs in lead at approximately room temperature. These hillocks are mechanical protrusions which develop on the surface of the lead electrodes. Because the tunnel barrier is so thin, it is quite easy to have these hillocks break through the tunnel barrier to cause shorts across the junction. This is particularly troublesome if large arrays of these devices are to be made, since the reliability of the array will depend upon the individual reliability of each junction device.
One previous attempt to provide improved Josephson junction devices is contained in Ser. No. 889,100, filed Dec. 30, 1969 and assigned to the present assignee. That application proposed the addition of certain impurities, e.g., alloying additions, to the thin metal film electrodes to negate the driving force (or the effect of the driving force) which causes hillock formation. Induced stresses in the superconducting electrodes cause atom movement via defect movement and hillock growth will result. The impurity additions in the electrodes affect hillock growth by their interaction with the defects.
Application Ser. No. 103,236, filed Dec. 31, 1970, now U.S. Pat. No. 3,733,526 describes an improved Josephson junction device in which indium or indium and tin underlayers are used in conjuction with lead electrodes. The indium and indium/tin diffuse into the lead to become uniform alloy additions in the base electrode. The resulting Josephson junction has an improved tunnel barrier, since the resulting oxide barriers are reproducibly well controlled and of small thickness. They are also very uniform and dense even at these very small thicknesses.
Whereas research is continuing in the problem of Josephson junctions having good thermal cycling and other characteristics, this research has not developed devices and methods of fabrication which will provide good devices no matter what materials are used for electrodes.
Accordingly, it is a primary object of this invention to provide a Josephson junction device having improved stability over repeated temperature cycles.
It is another object of this invention to provide a Josephson tunneling device having improved mechanical stability and high Josephson current.
Still another object of this invention is to provide a Josephson tunneling device having improved stability and resistance to hillock growth using a wide variety of superconducting materials.
A further object of this invention is to provide an improved Josephson tunneling device which is resistant to stress relaxation processes, such as hillock growth and which has good tunneling barriers.
The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of the preferred embodiments of the invention as illustrated in the accompanying drawings.